Pakwheeler (formerly known as upYours) is right about the HP the point ot note here is at the wheels. There are 2 places wher power is measured at the wheels (whp) and at the crank (hp). Pakwheeler is talking about at the wheels and if you look at what you said and what he said, you will know that both of you are right.
300hp - 15% loss of power in the drivetrain=255whp
Actually the loss of drivetrain power is between 15-25% loss of power. This is due to the fact that in the factory the hp is measured with the engine outside the vehicle and the loss of power in thr transmission is not accounted. In the modification process it becomes to tedious to take in the engine out measure the power and place it back in again. when doing a next upgrade you would have to do the same procdure all over again. So to svae all this effort they use dynos where the power is measured of the engine through the wheels with all the losses and hence makes things easier for modification. Then you have WHP! Hope I cleared things here.
@Hash4u & @kiani
Yes it is possible but not easy. Now think about it from the factory a B16b series engine in a Civic type R is rated at 187hp at the crank this is best what Honda Mugen could do without making the car illegal on the streets (yes any modification makes your car illegal depending on the rules of the city you are in). B16a is rated at 160-170hp and to get 400hp is very very hard in N/A mode, so ppl turbo charged it and the most used turbo on a B series is T3/T4 these kits are found easier. But if you were to just use it on a stock engine you would have to stay aorund 6-7psi boost just so you do not stay away from melting your pistons and blowing the up the engine. In order to get more hp, forged pistons and connecting rods are used, the engine block is sleeved for strength or block guards are used (since B series is a open deck engine, simply put weeker) Honda uses this method of casting coz the open-deck design is less prone to casting reject, and its productivity is higher (see pic).
Other then this the compression ratio is lowered to 9:1 or lower for street turbo use so the setup can be turbo charged, with higher compressions the engine is prone to preignition or detonation. This is known as knocking which results from the violent explosion when the normal flame front runs into the secondary flame front. Detonation will cause piston and ring damage, top ring groove wear, scoring, sticking rings, loose head gaskets and possible complete engine failure. Lower CR will permit higher boost with optimized timing on low octane fuel. Forged pistons are an excellent idea on turbos as they have 2-3 times the strength and heat dissipation of cast pistons. Forged connecting rods, colder spark plugs and stronger head gaskets are also recommended.
In order to produce 400hp out of b16 lot of work need to be done and a bigger turbo setup need to be placed, i do not believe you can produce this figure with a T3/T4 turbo, you will probably need a Garret GT series twin ball bearing turbo. But only the Dyno can tell you the truth if that engine is producing 400hp, so look for dyno charts or figures. So yes it can be done but its not easy, lot of money and vehicle down time needed. And the guy should know what he is doing. But once achieved the satisfaction is amazing but it does not stop there.